New research studies the role of boreal forest emissions on cirrus clouds formation

The formation of ice crystals in cirrus clouds is triggered by the presence of aerosols, small particles in suspension in the atmosphere. Aerosols come from multiple sources, for example dust, fly ash, sea spray or pollen, but they can also be formed (secondary aerosols) in the atmosphere by the condensation of different vapors of natural or anthropogenic origin. Above boreal forests, secondary organic aerosol (SOA) particles are formed from tree emissions. Investigating the role of these particles in the formation of high altitude clouds is key to better understand the properties of clouds in boreal environments, like Finland.

In our research, we studied whether the cirrus clouds form more easily in the presence of SOA particles. Our laboratory-based research was conducted in collaboration with University of Eastern Finland (Aerosol Physics Research Group). We made boreal-forest-like particles from various precursors (α-pinene, pine needle oil, and real tree emissions) with different water content: dry (≤10% Relative Humidity) or wet (40% RH), to mimic what happens over boreal forests. These particles were exposed to cirrus cloud conditions to measure whether they help to form ice crystals.

We found that only when the water content of the particles is low these particles can form an ice crystal, and even then they are not more effective than a grain of salt that forms a solution droplet which then freezes (homogeneous nucleation). Our research shows that SOA from boreal forests do not facilitate the formation of cirrus clouds.

More information:

Research Scientist Ana Álvarez Piedehierro, ana.alvarez.piedehierro@fmi.fi

Leader of the project: Ari Laaksonen, ari.laaksonen@fmi.fi

Piedehierro, A. A., Welti, A., Buchholz, A., Korhonen, K., Pullinen, I., Summanen, I., Virtanen, A., and Laaksonen, A.: Ice nucleation on surrogates of boreal forest SOA particles: effect of water content and oxidative age, Atmos. Chem. Phys., 21, 11069–11078, 2021.

The scientific article can be read from here